1. Field of the Invention
The invention relates to the technical field of chemical processes for preparing intermediates which can be employed for preparing active compounds, other intermediates or products intended for use in various areas, for example in the areas of pharmaceutics, crop protection, polymers and/or dyes.
2. Description of the Related Art
The compound 2-carboxy-5-nitrobenzenesulfonic acid is described as an intermediate for the synthesis of various active compounds in U.S. Pat. No. 4,608,205, DE-A-4,334,949, DE-A4,236,902 and JP-A-57,036,121 and in chemistry handbooks. The preparation of the intermediate can be carried out, for example, by oxidation of 2-methyl-5-nitrobenzenesulfonic acid, and specific mention may be made of the reactions with hot basic potassium permanganate solution (see U.S. Pat. No. 4,608,205) with nitric acid at 130-220.degree. C. (see U.S. Pat. No. 2,675,405), with atmospheric oxygen in the presence of manganese salts and/or cobalt salts (see U.S. Pat. No. 5,175,351, CS-A-149,998, CS 150,000) or with hypochlorite in the presence of heavy metal peroxides and alkali metal hydroxides (see JP-A-57,200,353). The abovementioned oxidation processes have disadvantages which manifest themselves particularly strongly on scale-up, for example to batches on an industrial scale.
In addition to the problems related to processing and waste which are associated with the handling of large amounts of KMnO.sub.4, side reactions occur during this basic oxidation of 2-methyl-5-nitrobenzenesulfonic acid which considerably reduce the yield, such as the well-documented rapid dimerization of the starting material; see Chem. Ber. 1886 (19) 3234, Chem. Berichte 1897 (30) 3236 and J. Am. Chem. Soc. 1954 (76) 2725.
The oxidation with nitric acid, on the other hand, which avoids the base-induced dimerization, is, at the high reaction temperatures required, exothermic to such an extent that its practice on an industrial scale requires considerable control and specific precautions have to be taken to minimise the risk.
The slightly basic oxidations with hypochlorite (JP-57200353) or air (U.S. Pat. No. 5,175,351), however, require heavy metals as catalysts and do likewise have disadvantages, owing to the related expenditure for work-up and waste disposal. Additionally, peroxides may be formed which pose a risk of explosion. A risk of explosion in the abovementioned oxidations with air is also associated with the use of organic solvents on an industrial scale.
It is already known from U.S. Pat. No. 5,565,608 that the isomeric 4-methyl-3-nitrobenzenesulfonic acid (=2-nitro-4-tolyenesulfonic acid) can be readily oxidized on a laboratory scale to give the isomeric 4-carboxyl-3-nitrobenzenesulfonic acid (=2-nitro-4-sulfobenzoic acid) when the oxidation is carried out with NaOCl (sodium hypochlorite) at 50-150.degree. C., preferably at 100-110.degree. C., without heavy metal oxides. Surprisingly, this oxidation does not succeed when the conditions described therein are applied in an analogous manner to 2-methyl-5-nitrobenzenesulfonic acid. Even at temperatures (for example 45-50.degree. C.) below the reaction temperatures required for the oxidation, strong evolution of gas sets in, so that the reaction cannot be carried out in reaction vessels of economically viable size.
At higher temperatures (for example reflux temperature), evolution of gas is reduced, but the oxidation of the sodium or potassium salt of 2-methyl-5-nitrobenzenesulfonic acid with NaOCl under reflux conditions is considerably slower than the decomposition of the reagent. Thus, the reaction with 6 equivalents of aqueous sodium hypochlorite solution (13.5% by weight of NaOCl) gives only 45% conversion to the desired carboxylic acid. As already mentioned, it is additionally known that the oxidation with basic sodium hypochlorite solution gives the dimer 4,4'-dinitrobibenzyl-2,2'-disulfonic acid or 4,4'-dinitrostilbene-2,2'-disulfonic acid as byproducts (see DE-A-115410, DE-A-106961 and Chem. Ber. 1897 (30) 3097).
Other oxidations of methyl-substituted aromatics using sodium hypochlorite are known (see U.S. Pat. No. 3,790,624 and WO-9427959); however, in these oxidations heavy metals are employed as catalysts.
It is therefore an object of the invention to provide a process which allows the oxidation of 2-methyl-5-nitrobenzenesulfonic acid on an industrial scale without the abovementioned disadvantages.